1. Field of the Invention
The present invention relates to a driver for driving a light emitting element such as a semiconductor laser.
2. Related Background Art
In a laser beam printer, various drivers for driving the semiconductor laser have been used.
FIG. 4 shows a prior art driver for a laser diode in a constant current differential switching system. It is, for example, a differential current switch like that shown in Japanese patent Application Laid-Open No. 57-96583.
In FIG. 4, a constant current circuit comprises a power supply 12, an operational amplifier 10, a resistor 18 and a transistor 6. A collector current I.sub.LD of transistor 6 in the constant current circuit is expressed by the following formula, as shown in FIG. 6 EQU I.sub.LD =(voltage V4 of the power supply 12)/ (resistance of the resistor 18)
the base potential of transistor 3 is set to a constant potential (V3). The base potential P10 of the transistor 2 is set by resistors 26, 27 and 30 such that it is higher than the potential V3 when an output of an open-collector output AND gate 11 is "1" and lower than the voltage V3 when the output of the AND gate 11 is "0".
When input signals to the AND gate 11 are "1", the output of the AND gate 11 is also "1" and the base potential P10 of transistor 2 is higher than the base potential V3 of transistor 3 so that transistor 2 is turned on and transistor 3 is turned off. Accordingly, no current flows through the laser diode 1. In this case, a collector potential of the transistor 6 is equal to (base potential P10 of the transistor 2)-(V.sub.BE), where V.sub.BE is a base-emitter voltage of the transistor 2.
On the other hand, when the input signals to the AND gate 11 are "0", the output of the AND gate 11 is also "0" and base potential P10 of the transistor 2 is lower than base potential V3 of transistor 3 so that transistor 2 is turned off and transistor 3 is turned on. Accordingly, a current determined by the output of the constant current circuit flows through the laser diode 1. In this case, the collector potential Pll of transistor 6 is equal to (potential V3)-(V.sub.BE)
However, in this circuit, base potential P10 of transistor 2 changes as shown in FIG. 5(b) each time the input signals to the AND gate 11 change as shown in FIG. 5(a). Since the collector potential P11 of the transistor 6 changes as base potential P10 of the transistor 2 changes, the collector potential P11 of the transistor 6 oscillates as shown in FIG. 5(c) and the output current of the constant current circuit varies accordingly. Thus, the current I.sub.LD flowing through the laser diode 1 oscillates (overshoots) as shown in FIG. 5(d).
As seen from a current-light intensity characteristic of the laser diode 1 shown in FIG. 7, even a small change in the current (.DELTA.I.sub.LD) causes a large change in the light intensity (.DELTA.P.sub.LD).
Particularly in the laser printer, image quality is degraded by the overshoot, while an excess current flow shortens the life of the laser diode 1.
On the other hand, the overshoot may be absorbed by using capacitors and coils but this decreases the switching speed.